NDA (II) 2023 General Ability Test · Question 90 of 146

Correct — A, Litmus is neutral towards ethanol. Ethanol is an alcohol, and in water it does not release hydrogen ions in any amount an indicator can register, so neither strip of litmus changes. The classroom demonstration this question comes from is the paired test in which ethanoic acid and ethanol are each brought to litmus — the acid turns blue litmus red, the alcohol leaves both papers as they were. Ethanol is not entirely without acidic character: it gives up its hydroxyl hydrogen to sodium metal, forming sodium ethoxide and hydrogen gas. But that reaction needs a metal as reactive as sodium precisely because ethanol is a far weaker acid than water, and nothing that weak can shift litmus.

• (b)Ethanol turns blue litmus to red; confirming acidic nature of ethanol — This is the result for ethanoic acid, not ethanol. A carboxylic acid ionises in water to give hydrogen ions and turns blue litmus red; the hydroxyl group of an alcohol does not ionise appreciably, so no colour change occurs.
• (c)Ethanol turns red litmus to blue; confirming basic nature of ethanol — Ethanol is not a base. Turning red litmus blue requires hydroxide ions or an equivalent proton acceptor, such as sodium hydroxide or ammonia solution, and ethanol supplies neither.
• (d)Ethanol decolorizes litmus through bleaching action — Bleaching is what chlorine water, sulphur dioxide or a hypochlorite does — it destroys the dye rather than changing its colour. Ethanol has no oxidising or reducing power of that kind, so the litmus dye is left intact.

Litmus is a dye obtained from lichens that answers only one question — whether the solution touching it has an excess of hydrogen ions or of hydroxide ions. Alcohols sit almost exactly in the middle. The hydroxyl group of ethanol holds its hydrogen far more tightly than the carboxyl group of a carboxylic acid does, and ethanol is in fact a slightly weaker acid than water itself, so a solution of ethanol is effectively neutral to any ordinary indicator.

The whole item hinges on not sliding from ethanol to ethanoic acid. Both start from the same two carbon atoms and appear in the same textbook chapter; one is the alcohol in spirits and the other is the acid in vinegar. Options (b) and (c) are the two possible ways of getting the confusion wrong, and option (d) borrows the language of bleaching to catch a student who half-remembers that ethanol is used as a disinfectant. Distinguishing the two compounds by test rather than by memory is the safer habit — litmus separates them straight away, and so does the fizz of carbon dioxide when a carbonate is added to the acid but not to the alcohol.

• Litmus is a lichen-derived indicator; blue litmus turns red in acid and red litmus turns blue in a base.
• Ethanol does not change either colour of litmus, so it is treated as neutral.
• Ethanoic acid, the acid in vinegar, does turn blue litmus red, which is the standard contrast drawn with ethanol.
• Ethanol reacts with sodium metal to give sodium ethoxide and hydrogen, showing only an extremely weak acidic character — weaker than that of water.

• Reading ethanol as ethanoic acid — the two names differ by three letters and give opposite litmus results.
• Treating the sodium reaction as proof that ethanol is acidic enough to affect an indicator; it is not.
• Confusing an indicator colour change with bleaching, which destroys the dye instead of switching it.

Asked as a single-step laboratory-observation item — predict what an indicator does to a named compound.

• practice — not a real PYQ

  Which one of the following will turn blue litmus red?

  • (a)Ethanol
  • (b)Ethanoic acid
  • (c)Sodium hydroxide solution
  • (d)Sodium chloride solution

  Answer(b) Ethanoic acid — the carboxyl group ionises in water to release hydrogen ions, while ethanol and common salt solution are neutral and sodium hydroxide is basic.

• practice — not a real PYQ

  Ethanol reacts with sodium metal to give hydrogen gas and

  • (a)sodium ethanoate
  • (b)sodium ethoxide
  • (c)sodium carbonate
  • (d)ethene

  Answer(b) sodium ethoxide — sodium displaces the hydrogen of the hydroxyl group, which is the only sense in which ethanol behaves as an acid.